1. Field of the Invention
The present invention generally relates to multiple communication systems, exchanges and terminal devices and, more particularly, to a multiple communication system composed of a plurality of communication networks including mobile communication networks, and to exchanges and terminal devices used in such a system, capable of efficiently processing messages incoming to mobile terminal devices moving in the network.
2. Description of the Related Art
A description will now be given of the related art. FIG. 9 is a system block diagram of a multiple communication system. Communication networks constituting a multiple communication system include point-to-point communication networks and mobile communication networks. In FIG. 9, numeral 1 indicates a public telephone network (point-to-point communication network), 2 and 3 indicate cellular communication networks (mobile communication networks), and 4 indicates a satellite communication network (mobile communication network). The coverage of each of the communication networks may or may not overlap geographically. Numeral 10 indicates a public exchange, 20 and 30 indicate cellular exchanges and 40 indicates a satellite exchange. Exchanges are classified into three categories according to the function thereof: gateway exchanges that serve as gateways connecting a home communication network with another communication network; subscriber exchanges connected to subscriber terminal devices; and relay exchanges for connecting and relaying between a plurality of subscriber exchanges. A generic term of "exchange" will be used if the type of the communication network to which the exchange belongs and the function thereof are to be disregarded. The public telephone network 1, the cellular communication networks 2, 3, and the satellite communication network 4 are connected with each other via the exchanges. It is assumed that the exchanges shown in FIG. 9 include the functions of a gateway exchange and a subscriber exchange. The public exchange 10, the cellular exchanges 20, 30, and the satellite exchange 40 are similarly constructed. The public exchange 10 is comprised of a control circuit 11 and a memory 12; the cellular exchange 20 is comprised of a control circuit 21 and a memory 22; the cellular exchange 30 is comprised of a control circuit 31 and a memory 32; and the satellite exchange 40 is comprised of a control circuit 41 and a memory 42. Numeral 13 indicates a terminal device which belongs to the public telephone network 1, and 23 indicates a dual-mode mobile terminal device which belongs to the mobile communication network. The dual-mode mobile terminal device is adapted to a plurality of communication modes and can communicate even when it moves to a mobile communication network other than the home mobile communication network to which it originally belongs.
FIG.10 is a flow chart showing the operation of the related-art multiple communication system. A description will be given of the operation of the related-art multiple communication system with reference to this flow chart. It is assumed that the home communication network for the dual-mode mobile terminal device 23 is the cellular communication network 2, and that the dual-mode mobile terminal device 23 is away from the home network and moving to the satellite communication network 4. The position registration information of the dual-mode mobile terminal device 23 is registered in the memory 42 of the satellite exchange 40 in the destination satellite communication network 4 as well as in the memory 22 of the cellular exchange 20 in the home cellular communication network 2. It is assumed that terminal device 13 issues a call to the dual-mode mobile terminal device 23. When a connection request signal from the terminal device 13 is sent to the public exchange 10 in the public telephone network 1, the public exchange 10 recognizes that the home of the mobile terminal device 23 is the cellular communication network 2 by referring to the telephone number information contained in the connection request signal so as to forward the connection request signal to the cellular exchange 20 (step ST401). The cellular exchange 20 examines the memory 22 and, if it is determined that the dual-mode mobile terminal device 23 is located in the cellular communication network 2, processes the incoming call. When the dual-mode mobile terminal device 23 is not engaged in a call so that the call incoming process is successful, control is turned to a speech session mode (steps ST402 and ST403). If the dual-mode mobile terminal device 23 is located in the home cellular communication network 2 but a telephone call is already proceeding or the power supply is turned off, the call incoming process is unsuccessful (steps ST402 and ST404). When the dual-mode mobile terminal device 23 is away from the home network and moving to the satellite communication network 4, as assumed in this case, the cellular exchange 20 recognizes that the destination for connection is the satellite communication network 4, based on the position registration information which is related to the telephone number information of the dual-mode mobile terminal device and which is registered in the memory 22. Thereupon, the cellular exchange 20 informs the public exchange 10 of the destination for connection. The public exchange 10 then sends the connection request signal to the satellite exchange 40 (step ST405). The satellite exchange 40 forwards the connection request signal to the dual-mode mobile terminal device 23 so that a call incoming process is performed. When the call incoming process is successful, control is turned to a speech session mode (steps ST406 and ST407). If the call incoming process is unsuccessful, notification of a failure of call incoming is returned to the call originating terminal device 13 (steps ST406 and ST408).
Japanese Laid-Open Patent Application No. 57-99853 discloses a traffic control method designed to evade overcrowding of a terrestrial circuit in an area where the coverage of the terrestrial circuit and a satellite circuit overlap, by selecting only the satell ite circuit for calls identified by a designated call incoming code even when connection via the terrestrial circuit is available.
A disadvantage of the related-art multiple communication system is that the exchange in the home communication network is examined first .According to this approach, when the mobile terminal device is away from the home communication network, it takes a relatively long period of time before the connection is established due to a complicated intervening network construction comprised of a plurality of terrestrial networks and satellite networks. The method disclosed in Japanese Laid-Open Patent Application No. 57-99853 is designed to evade overcrowding of communication in an area where the coverage of a communication line of one type overlaps that of a communication of another type. For this reason, Japanese laid-Open Patent Application No. 57-99853 fails to resolve the problem of an extended period of time needed to establish a connection. The designated call incoming code according to the related art merely enables selection of the satellite network. Therefore, users cannot select a type of connection suited to their needs. That is, users cannot select options like "terrestrial network only", "priority to satellite network" or "priority to terrestrial network".